Divalent Ion Removal From Monoethylene Glycol-Water Streams

ABSTRACT

A system and process for the removal of divalent ions from a MEG-water stream are presented. The system includes a chemical treatment tank that receives the MEG-water stream, means for precipitating the calcium, magnesium, and hydroxide ions that are contained in the MEG-water stream, means for discharging the MEG-water stream from the chemical treatment tank, and means for recycling the discharged MEG-water stream back to the chemical treatment tank or routing it to a solids removal system. The means for precipitating the calcium, magnesium, and sulfate ions may be employed simultaneously or sequentially.

BACKGROUND

This invention relates to systems and processes designed to treatmonoethylene glycol (MEG) used in the oil and gas industry, especiallyin offshore locations, to control the formation of hydrates. Moreparticularly, the invention relates to systems and processes that aredesigned for the simultaneous removal of divalent cations and sulfatefrom MEG-water streams through the addition of alkalinity and bariumions.

In the oil and gas industry, MEG is widely used in wellheads andpipelines as a hydrate suppressor to prevent hydrate formation atpipeline conditions. On offshore gas production facilities, where theexposure to lower temperatures in subsea pipelines is significant, MEGis in prevalent use for hydrate inhibition. The lean (dry) MEG isinjected in the subsea gas pipeline at or near the wellhead and mixesreadily with the produced water. The inhibition process isstraightforward, with the MEG decreasing the hydrate formationtemperature below the operating temperature and thus preventing hydrateblockage of the pipeline.

After use, the MEG is recovered by removing the water and the dissolvedsalts, which are produced from the well along with the gas. The removalof water is conventionally referred to as MEG regeneration, while theremoval of the dissolved salts is conventionally known as MEGreclamation. After regeneration and reclamation, the MEG can be re-usedin hydrate control.

If the dissolved salts are not removed, they can form scale in pipelinesand in processing equipment. The extent of scaling depends on theconcentrations of the ions and process conditions such as temperature,pressure, and the concentration of carbon dioxide. As an example, as theMEG-water stream passes through MEG regeneration, the temperature israised and scale may form from Na₂SO₄, CaCO₃ from unprecipitated calciumions, and Mg(OH)₂ from unprecipitated magnesium ions. Scaling may reducethe efficiency of flow through the pipelines and cause the failure ofdownstream treatment processes.

Divalent cations can be removed by adding alkalinity in the form ofcarbonate ions, hydroxide ions, or both to raise the pH of the solution,which causes the divalent cations to precipitate as insoluble carbonatesor hydroxides. However, raising the pH does not remove the sulfate ions.As a result, when the lean MEG is re-injected into the gas productionpipeline and mixed with the produced water, the sulfate ions in the leanMEG and the calcium, magnesium, and sulfate ions in the produced watermay combine to form scale in the pipeline.

A need exists for systems and processes for removing divalent ions fromMEG-water streams in order to improve the efficiency of MEG reclamationand MEG regeneration and to prevent the accumulation of scale inside gasproduction pipelines and process equipment.

SUMMARY OF THE INVENTION

A system for the removal of divalent ions from a MEG-water stream ispresented. The system includes a chemical treatment tank that receivesthe MEG-water stream, means for precipitating the calcium, magnesium,and sulfate ions that are contained in the MEG-water stream, means fordischarging the MEG-water stream from the chemical treatment tank, andmeans for recycling the discharged MEG-water stream back to the chemicaltreatment tank or routing it to a solids removal system. The means forprecipitating the calcium, magnesium, and sulfate ions may be employedsimultaneously or sequentially.

A process for the removal of divalent ions from a MEG-water stream isalso presented. The process includes the steps of routing a MEG-waterstream containing calcium, magnesium, and sulfate ions to a chemicaltreatment tank, precipitating the sulfate ions in the MEG-water stream,precipitating the calcium ions in the MEG-water stream, precipitatingthe magnesium ions in the MEG-water stream, and discharging theMEG-water stream from the chemical treatment tank. If the precipitationsteps occur simultaneously, the MEG-water stream is recycled to thechemical treatment tank until the precipitation reactions are completeand then routed to a solids removal system and filtrate tank for furtherprocessing. If the precipitation steps occur sequentially, the MEG-waterstream is recycled to the chemical treatment tank until theprecipitation reaction for barium sulfate is complete. The MEG-waterstream is then routed to a solids removal system, where the bariumsulfate is separated for disposal, and then recycled to the chemicaltreatment tank. The process is repeated for the step of precipitatingthe calcium ions and the magnesium ions.

The objects of this invention include (1) providing a more efficientprocess to remove divalent ions contained in a MEG-water stream; (2)providing for the quantitative removal of divalent ions and sulfate ionsin the same treatment process; (3) minimizing the formation of scale inpipelines and downstream treatment processes; (4) reducing the need forthe use of clean-in-place systems and scale inhibitors; and (5) reducingthe amount of time the process equipment must be taken off-line forcleaning.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a preferred embodiment of a system and process practicedaccording to this invention.

FIG. 2 is a schematic of an apparatus used to test preferred embodimentsof the system and process of this invention.

FIG. 3 shows experimental results of a system and process made accordingto FIG. 1 for the removal of magnesium, calcium, barium, and sulfateions from a first experimental test solution.

FIG. 4 shows experimental results of a system and process made accordingto FIG. 1 for the removal of magnesium, calcium, barium, and sulfateions from a first experimental test solution, according to the removalefficiency in percent for each ion.

FIG. 5 shows experimental results of a system and process made accordingto FIG. 1 for the removal of magnesium, calcium, barium, and sulfateions from a first experimental test solution. The calculatedprecipitation efficiency for each ion is plotted against the measured pHof the test solution.

FIG. 6 shows experimental results of a system and process made accordingto FIG. 1 for the removal of magnesium, calcium, barium, and sulfateions from a second experimental test solution.

FIG. 7 shows experimental results of a system and process made accordingto FIG. 1 for the removal of magnesium, calcium, barium, and sulfateions from a second experimental test solution, according to the removalefficiency in percent for each ion.

ELEMENTS AND NUMBERING USED IN THE DRAWINGS AND THE DETAILED DESCRIPTION

-   5 Chemical treatment tank-   10 MEG-water stream-   20 Barium chloride injection line-   30 Sodium carbonate injection line-   40 Sodium hydroxide injection line-   50 Exit line from chemical treatment tank-   55 Mixing pump-   60 Recycle line to chemical treatment tank-   70 Line to solids removal system-   75 Solids removal system-   80 Recycle line to chemical treatment tank-   90 Solids disposal-   100 Filtrate/centrate from solids removal system-   110 Filtrate/centrate to filtrate tank-   115 Filtrate tank-   120 Filtrate/centrate recycle line to chemical treatment tank-   130 Exit line for MEG-water stream

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A system and process made and practiced according to this inventiontreats MEG-water streams so that the MEG can be re-used in the gasproduction pipeline system. More particularly, the system and processare designed for the simultaneous removal of divalent cations andsulfate from MEG-water streams through the addition of alkalinity andbarium ions. Divalent cations may include, but are not limited to,calcium, magnesium, iron, strontium, and barium. Depending upon theconcentration of divalent ions in the MEG-water stream, its pH, andother factors, the alkalinity may be a sodium carbonate solution, asodium hydroxide solution, a potassium carbonate solution, a potassiumhydroxide solution, or a combination of the above.

Referring to FIG. 1, a system for removing divalent ions from aMEG-water stream includes a chemical treatment tank 5 to receive theincoming MEG-water stream 10, which contains calcium, magnesium, andsulfate ions. The chemical treatment tank 5 also receives a bariumchloride solution through a barium chloride injection line 20, a sodiumcarbonate solution through a sodium carbonate injection line 30, and asodium hydroxide solution through a sodium hydroxide injection line 40.The weight percentage of barium chloride in the barium chloride solutionmay be approximately 20 percent at 25° C., the weight percentage ofsodium carbonate in the sodium carbonate solution may be approximately20 percent at 25° C., and the weight percentage of sodium hydroxide inthe sodium hydroxide solution may be approximately 47 percent at 25° C.Potassium carbonate may be used as an alternative to sodium carbonate,and potassium hydroxide may be used as an alternative to sodiumhydroxide.

After mixing, the MEG-water stream is carried away from the chemicaltreatment tank 5 by an exit line 50. The MEG-water stream may be pumpedby a mixing pump 55 through recycle lines 60, 80 to the chemicaltreatment tank 5. Alternatively, the MEG-water stream may be pumpedthrough line 70 to a solids removal system 75, where the precipitatedsolids are removed and sent for disposal 90. Solids removal systems suchas filters and centrifuges are well-known in the art, and any suitablesystem may be used with the invention. The filtrate or centrate from thesolids removal system 75 exits the system through line 100. From there,it may be recycled through line 120 to the chemical treatment tank 5 orsent through line 110 to the filtrate tank 115. The MEG-water streamfrom the filtrate tank 115 is then sent through exit line 130 todownstream equipment for additional treatment.

The precipitation of divalent ions and sulfate may occur simultaneouslyor sequentially. For simultaneous precipitation, the MEG-water stream 10enters the chemical treatment tank 5. Barium chloride, sodium carbonate,and sodium hydroxide solutions are added to the chemical treatment tank5 through lines 20, 30, and 40, respectively, at individual ratesdetermined by the flow and characteristics of the MEG-water stream 10.The resulting mixture exits the chemical treatment tank 5 through line50 and is recycled back to the tank 5 through lines 60 and 80 until theprecipitation reactions are complete. The MEG-water stream 10 is thenrouted to the solids removal system 75 through line 70. The solids aresent for disposal 90, while the filtrate or centrate exits the solidsremoval system 75 through line 100. The filtrate or centrate is thenrouted to the filtrate tank 115 through line 110 and subsequently on toMEG regeneration.

For sequential precipitation, the MEG-water stream 10 enters thechemical treatment tank 5. A barium chloride solution is added to thechemical treatment tank 5 through line 20 at a rate determined by theflow and characteristics of the MEG-water stream. The resulting mixtureexits the chemical treatment tank 5 through line 50 and is recycled backto the tank 5 through lines 60 and 80 until the precipitation reactionfor barium sulfate is complete. The stream is then routed to the solidsremoval system 75 through line 70. The barium sulfate solids are sentfor disposal 90, while the filtrate or centrate exits the solids removalsystem 75 through line 100 and is routed to the chemical treatment tank5 through line 120. The process is repeated a second time for theaddition of a sodium carbonate solution through line 30, which resultsin the precipitation of the calcium ions as calcium carbonate, and theaddition of a sodium hydroxide solution through line 40, which resultsin the precipitation of the magnesium ions as magnesium hydroxide.Alternatively, the process is repeated a second time for the addition ofa sodium carbonate solution through line 30, which results in theprecipitation of the calcium ions as calcium carbonate, and a third timefor the addition of a sodium hydroxide solution through line 40, whichresults in the precipitation of the magnesium ions as magnesiumhydroxide. The individual rates of addition for the sodium carbonate andsodium hydroxide solutions are determined by the flow andcharacteristics of the MEG-water stream 10. After all the solids havebeen separated for disposal 90 by the solids removal system 75, theremaining filtrate or centrate is routed to the filtrate tank 115through line 110 for further processing.

Experimental Results

The equipment used to test the simultaneous precipitation of magnesium,calcium, and sulfate ions is shown in FIG. 2. The equipment includes adouble-skinned 5 L glass reaction vessel with a stirrer. The reactionvessel is connected to a hot oil circulator bath which allows thetemperature of the reaction vessel to be adjusted between −10° C. and150° C. The reaction vessel is fitted with a pH probe (Hamilton PolilytePlus ARC 425), dissolved oxygen probe (Hamilton Oxygold G ARC 425),electrical conductivity probe (Hamilton Conducell 4USF ARC PG425), RedOxprobe (Hamilton Polilyte Plus ORP ARC 425), and a thermocouple (notshown) to measure liquid temperature during the progression of the test.A small flow of nitrogen (100 mL/min) was passed through the vapor spaceabove the liquid inventory. Liquids were added to the reaction vesselthrough nozzles located at the top of the vessel. All samples were takenthrough a drain point at the base of the reaction vessel.

Sequential Addition of BaCl₂, Na₂CO₃, and NaOH

A first experimental test solution as shown in TABLE 1 was loaded intothe reaction vessel.

TABLE 1 First Experimental Test Solution Water MEG NaCl MgCl₂ CaCl₂Na₂SO₄ NaHCO₃ grams 1,593 2,349 129 1.83 17.33 2.93 1.74 mmoles — —2,207 19.2 156.1 20.6 20.7

The test solution was heated to 30° C. when a first sample was taken foranalyses (SAMPLE 1, approximately 30 grams). Dissolved cations (Na⁺, K⁺,Mg²⁺, Ca²⁺, Fe²⁺, Sr²⁺, Ba²⁺) were measured using Inductively-CoupledPlasma Optical Emission Spectroscopy (ICP-OES). Dissolved anions (Cl⁻,Br⁻, SO₄ ²⁻) were measured using Ion Chromatography (IC). The testsolution was then heated to 61.5° C. and a second sample was taken(SAMPLE 2, approximately 30 grams). 20.79 grams of 20 wt % bariumchloride solution (20 wt % BaCl₂ in water, equivalent to 4.158 grams ofbarium chloride, 20.0 mmoles) were then added to the reaction vessel.The resulting solution was allowed to mix for fifteen minutes and athird sample was taken (SAMPLE 3, approximately 30 grams).

90.93 grams of 20 wt % sodium carbonate solution (20 wt % Na₂CO₃ inwater, equivalent to 18.19 grams of sodium carbonate, 171.6 mmoles) wereadded to the reaction vessel and a fourth sample was taken (SAMPLE 4). Afurther 10 grams of 20 wt % sodium carbonate solution were added toelevate the solution pH to 9.5 and a fifth sample was taken (SAMPLE 5).The total amount of sodium carbonate added to the first experimentaltest solution was 20.19 grams or 190.4 mmoles.

3.20 grams of 50 wt % sodium hydroxide solution (50 wt % NaOH in water,containing 1.6 g of sodium hydroxide, 40 mmoles) were added to thereaction vessel, which elevated the pH of the first experimental testsolution to 10.0, and a sixth sample was taken (SAMPLE 6). Finally, anadditional 8.56 grams of 50 wt % sodium hydroxide solution were added tothe reaction vessel to elevate the pH to 10.59 and a seventh sample wastaken (SAMPLE 7). The total amount of sodium hydroxide added to thefirst experimental test solution was 5.88 grams or 147.0 mmoles.

Analytical results for SAMPLES 1-7 are shown in FIG. 3 and TABLE 2.

TABLE 2 Analytical Results SAMPLE # — 1 2 3 4 5 6 7 BaCl₂ AQ g — — 20.820.8 20.8 20.8 20.8 (20%) Na₂CO₃ AQ g — — — 90.9 100.2 100.2 100.2 (20wt %) NaOH AQ g — — — — — 3.2 11.8 (20 wt %) TEMP degC 30 60 61.5 60.460.4 60.4 60.4 pH — 6.97 7.28 6.91 8.99 9.54 10 10.59 Na mg/kg 12,40012,300 12,400 13,900 13,900 14,200 14,600 K mg/kg <25 <25 <25 <25 <25<25 <25 Mg mg/kg 105 111 110 35 27 11 0.3 Ca mg/kg 1500 1500 1470 6.43.1 2.5 0.1 Fe mg/kg <0.7 <0.7 <0.7 <0.7 <0.7 <0.7 <0.7 Sr mg/kg 0.630.63 0.4 <0.04 <0.04 <0.04 <0.04 Ba mg/kg <0.1 <0.1 31.7 0.43 4 3.2 0.22Zn mg/kg 0.65 0.49 0.6 0.51 <0.4 <0.4 0.6 S mg/kg 162 165 <18 <18 <18<18 19 Cl mg/L 30,744 23,191 22,885 22,502 21,545 22,332 22,667 Br mg/L4.4 4.4 4.4 4.3 4.4 4.6 4.5 NO₃ mg/L 3.1 3.1 3.2 3 3 2.9 2.9 PO₄ mg/L5.7 5.1 5.1 5.1 5.4 5.2 8.8 SO₄ mg/L 454.9 467.1 10 30.9 14.9 15.4 61.5

Removal efficiencies for magnesium, calcium, barium, and sulfate ionsare shown in FIG. 4 and TABLE 3. The removal efficiency is calculatedusing the following formula:

$\begin{matrix}\text{Removal} \\\text{Efficiency~~of} \\{\text{Species~~}X}\end{matrix} = \frac{\begin{matrix}{{\text{Concentration~~of~~species~~}X\text{~~in feed}\mspace{14mu} ({ppm})} -} \\{{Concentration}\mspace{14mu} {of}\mspace{14mu} {species}\mspace{14mu} X\mspace{14mu} {in}\mspace{14mu} {sample}\mspace{14mu} ({ppm})}\end{matrix}}{{Concentration}\mspace{14mu} {of}\mspace{14mu} {species}\mspace{14mu} X\mspace{14mu} {in}\mspace{14mu} {feed}\mspace{14mu} ({ppm})}$X = Mg²⁺, Ca²⁺, SO₄²⁻

For barium removal, the “concentration of species X in feed” was basedon the quantity of barium chloride solution added to the test solution.

TABLE 3 Calculated M²⁺ and SO₄ ²⁻ Removal Efficiencies SAMPLE # — 1 2 34 5 6 7 BaCl₂ AQ g — — 20.8 20.8 20.8 20.8 20.8 (20%) Na2CO₃ AQ g — — —90.9 100.2 100.2 100.2 (20 wt %) NaOH AQ g — — — — — 3.2 11.8 (20 wt %)TEMP degC 30 60 61.5 60.4 60.4 60.4 60.4 pH — 6.97 7.28 6.91 8.99 9.5410 10.59 Mg % PPTD — — — — 20.1 67.5 99.1 Ca % PPTD — — 1.5 99.6 99.899.8 100.0 Ba % PPTD — 100.0 99.4 99.9 99.4 99.5 100.0 SO₄ % PPTD — 3.697.9 93.6 96.9 96.8 87.3

As shown in TABLE 2 and FIG. 3, there was little precipitation from thefirst experimental test solution on original mixing (SAMPLE 1) and whenthe solution temperature was increased to 60° C. (SAMPLE 2). SAMPLE 3shows that the addition of barium chloride (4.16 grams, 20.0 mmoles)resulted in almost quantitative removal of the sulfate anions in thetest solution, with sulfate concentrations falling from 467 mg/L to 10mg/L (97.9% removal). The measured barium concentration increased from<0.1 mg/kg to 32 mg/kg. This corresponds to removal of 95.3% of thebarium added as barium chloride. However, calcium and magnesium levelsbefore and after the addition of barium chloride were unchanged.

The addition of alkalinity (as sodium carbonate and sodium hydroxide)resulted in quantitative removal of the calcium. At a pH of 9.0 (SAMPLE4), the measured calcium concentration decreased from 1,470 mg/kg to 6.4mg/kg. Calcium concentrations continued to decrease as the pH increased.The final calcium concentration, measured at a pH of 10.6 (SAMPLE 7),was 0.1 mg/kg, which corresponds to 99.99% calcium removal. The bariumpresent after the addition of barium chloride (31.7 mg/kg in SAMPLE 3)also precipitated as the pH increased. The final barium concentration(SAMPLE 7), was 0.22 mg/kg, which corresponds to 99.97% barium removal.

Because magnesium is more soluble in alkaline carbonate solutions thanboth calcium and barium, a higher pH is required to precipitatemagnesium from the test solution. At a pH of 9.0 (SAMPLE 4), themeasured magnesium concentration was 35 mg/kg, which corresponds toprecipitation of 69% of the magnesium originally present in thesolution. As the pH increased to 10.0, the magnesium concentrationdecreased to 11 mg/kg, which corresponds to 90.4% magnesiumprecipitation (SAMPLE 6). At a pH of 10.6, the magnesium concentrationwas 0.3 mg/kg, which corresponds to 99.7% magnesium removal (SAMPLE 7).

It can be seen from TABLES 2 and 3 that sulfate precipitation occurs atlow pH when barium chloride is added and that the sulfate remains in thesolid phase at pH up to 10.0. However, some re-dissolution of thesulfate occurs at higher pH. As an example, measured sulfate levelsvaried from 15 mg/L to 31 mg/L over the pH range of 9.0 to 10.0, whichcorresponds to sulfate precipitation of 94% to 97%. At a pH of 10.6, themeasured sulfate level was 60 mg/L, which corresponds to sulfateprecipitation of only 87%. FIG. 5, which plots the precipitation ofmagnesium, calcium, barium, and sulfate ions against the pH of the testsolution, further illustrates that the precipitation efficiency ofsulfate decreases at higher pH. Effective pH monitoring and control istherefore required in order to optimize the precipitation of divalentcations and sulfate.

Simultaneous Addition of BaCl₂, Na₂CO₃, and NaOH

A second experimental test solution was prepared as per TABLE 4.

TABLE 4 Second Experimental Test Solution Water MEG NaCl MgCl₂ CaCl₂Na₂SO₄ NaHCO₃ g 1,648 2,542 127 0.56 17.47 3.03 0 mmoles — — 2,173 5.88157.39 21.32 0Solutions of barium chloride, sodium carbonate, and sodium hydroxidewere prepared as per TABLES 5-7.

TABLE 5 Barium Chloride Solution BaCl₂.2H₂O BaCl₂.2H₂O WATER SOLUTIONBaCl₂ BaCl₂/Ba g mmoles g g wt% g 5.22 21.38 20.03 25.25 17.6 4.44/2.93

TABLE 6 Sodium Carbonate Solution Na₂CO₃ Na₂CO₃ WATER SOLUTION Na₂CO₃ gmmoles g g wt% 20.02 188.87 95.99 116.01 17.3

TABLE 7 Sodium Hydroxide Solution NaOH NaOH WATER SOLUTION NaOH g mmolesg g wt% 0.59 14.75 7.16 7.75 7.6

The second experimental test solution was heated at atmospheric pressureto 60° C. with continuous stirring. The barium chloride, sodiumcarbonate, and sodium hydroxide solutions were then added in 25%aliquots at 2.5-minute intervals and samples were taken as shown inTABLE 8 below.

TABLE 8 Chemical Addition ELAPSED BaCl₂ Na₂CO₃ NaOH Measured SAMPLE TIMEsoltn soltn soltn pH ID minutes g g G — — — 0 0 0 7.85 1 2.5 6.31 29 1.98.85 → 8.70 2 5 12.63 58 3.9 9.04 → 9.00 3 7.5 18.94 87 5.8 9.26 → 9.244 10 25.25 116 7.8 9.70 5 15 25.25 116 7.8 9.70 6 20 25.25 116 7.8 9.707 30 25.25 116 7.8 9.70 8 60 25.25 116 7.8 9.70 9The addition of the barium chloride, sodium carbonate, and sodiumhydroxide solutions to the second experimental test solution shown inTABLE 4 results in a solution containing 653 ppm of barium. Theanalytical results for SAMPLES 1-9 are shown in TABLE 9 and illustratedin FIG. 6.

TABLE 9 Analytical Results SAMPLE # — 1 2 3 4 5 6 7 8 9 Na mg/kg 11,40011,600 12,500 12,600 12,800 13,000 13,100 13,000 13,300 K mg/kg <30 <30<30 <30 <30 <30 <30 <30 <30 Mg mg/kg 33 31 26 14 0.6 1.2 1.7 1.3 1.2 Camg/kg 1,300 964 601 168 2 3 2.9 2.6 22 Fe mg/kg <0.7 <0.7 <0.7 <0.7 <0.7<0.7 <0.7 <0.7 <0.7 Sr mg/kg 0.58 0.3 0.14 0.06 <0.05 <0.05 <0.05 <0.05<0.05 Ba mg/kg 0.3 0.1 0.1 0.2 0.3 0.6 1.7 0.8 1.5 Zn mg/kg 0.6 0.6 0.80.9 1.2 0.6 0.7 <0.5 <0.5 S mg/kg 164 186 187 65 39 <20 <20 <20 <20 Clmg/L 22,394 22,084 21,317 20,989 22,417 20,499 19,424 21,201 21,134 Brmg/L 748 742 726 722 724 748 719 732 728 NO₃ mg/L 372 433 266 324 268240 186 338 163 PO₄ mg/L 801 832 853 802 815 787 787 794 793 SO₄ mg/L449 592 578 214 122 53 24 41 25TABLE 10 and FIG. 7 show the calculated removal efficiencies formagnesium, calcium, and sulfate from the original solution as well asfor the added barium.

TABLE 10 Precipitation Efficiency SAMPLE # — 1 2 3 4 5 6 7 8 9 Mg %precipitated — 3.1 18.8 56.3 98.1 96.3 94.7 95.9 96.3 Ca % precipitated— 31.8 57.5 88.1 99.9 99.8 99.8 99.8 99.8 Ba^([1]) % precipitated —100.0 100.0 100.0 100.0 99.9 99.7 99.9 99.8 SO₄ % precipitated — 53.473.4 88.4 94.6 91.1 94.6 ¹The barium precipitated refers to the bariumwhich was added to the test solution. The calcium and magnesium removalefficiencies refer to the calcium and magnesium originally present inthe test solution.As shown in the referenced tables and figures, the quantitative removalof sulfate, magnesium, and calcium ions can be achieved in twenty totwenty-five minutes at 60° C. by the addition of stoichiometricquantities of barium, hydroxide, and carbonate, respectively.

Simulated Results

Simulated results were obtained employing OLI Stream Analyzer™ software(OLI Systems, Inc., Cedar Knolls, N.J.). A solution without bicarbonate(First Model Test Solution, TABLE 11) was modeled for divalent cationand sulfate removal.

TABLE 11 First Model Test Solution kg mg/kg mol mmol/kg Water 1500 — — —Monoethylene 1500 — — — glycol Sodium chloride 100 32105 1710.9 549.3Magnesium 1.00 321 10.5 3.4 chloride Calcium chloride 13.00 4174 117.137.6 Sodium sulfate 0.80 257 5.6 1.8 Sodium bicarbonate 0.00 0 0.0 0.03114.8A second solution containing 770 mg/kg of sodium bicarbonate (SecondModel Test Solution, TABLE 12) was also modeled for divalent cation andsulfate removal.

TABLE 12 Second Model Test Solution kg mg/kg mol mmol/kg Water 1500 — —— Monoethylene glycol 1500 — — — Sodium chloride 100 32080 1710.9 32.08Magnesium chloride 1.00 321 10.5 0.32 Calcium chloride 13.00 4170 117.14.17 Sodium sulfate 0.80 257 5.6 0.26 Sodium bicarbonate 2.40 770 28.60.77 3117.2Barium chloride, sodium carbonate, and sodium hydroxide were added tothe test solutions in order to precipitate the sulfate, calcium, andmagnesium, respectively, according to the reactions shown below:

BaCl₂(aq)+Na₂SO₄(aq)→BaSO₄(s)+2NaCl(aq)

CaCl₂(aq)+Na₂CO₃(aq)→CaCO₃(s)+2NaCl(aq)

MgCl₂(aq)+2NaOH→Mg(OH)₂(s)+2NaCl(aq)

First Model Test Solution

The addition of barium chloride to the first model test solution isshown in TABLE 13. It can be seen that sulfate is removed almostquantitatively at Ba:SO₄=1:1 mol:mol.

TABLE 13 Addition of BaCl₂ to First Model Test Solution Solution ID 6.1.6.2. 6.3. 6.4. 6.5. 6.6. 6.7. Ba:SO₄ (mol:mol) 0.00 0.43 0.68 0.94 1.001.02 1.28 pH 5.92 5.90 5.89 5.88 5.88 5.88 5.87 Ba Precipitated % 0% 99%99% 98% 96% 95%  78% SO₄ Precipitated % 0% 42% 68% 92% 96% 97% 100%The solution after addition of barium chloride to achieve Ba²⁺:SO₄ ²⁻ of1:1 mol:mol was mixed with sodium carbonate (Solution 6.5, above). Theresults are shown in TABLE 14.

TABLE 14 Addition of Na₂CO₃ to First Model Test Solution after BaCl₂Addition Solution ID 7.1. 7.2. 7.3. 7.4. 7.5. 7.6. Ba:SO₄ (mol:mol)1.000 1.000 1.000 1.000 1.000 1.000 CO₃:Ca (mol:mol) 0.000 0.403 0.8060.967 1.000 1.100 pH 5.88 7.79 8.03 8.36 8.56 9.18 Ca Precipitated 0.0%40.1% 80.2% 95.8% 98.3% 99.9% Ba Precipitated 96.4% 96.8% 97.3% 97.5%97.6% 97.6% SO₄ Precipitated % 96.4% 96.8% 97.3% 97.5% 97.6% 97.6%

TABLE 14 shows that calcium precipitates from solution as the sodiumcarbonate is added and near quantitative calcium removal is achieved atCO₃ ²⁻:Ca²⁺=1.1:1 mol:mol (Solution 7.6. above, 99.9% calciumprecipitation). No re-dissolution of barium carbonate is predicted.

The solution after addition of barium chloride to achieve Ba²⁺:SO₄ ²⁻ of1:1 mol:mol and sodium carbonate addition at CO₃ ²⁻:Ca²⁺ at 1.1:1mol:mol was mixed with sodium hydroxide. The results are shown in TABLE15.

TABLE 15 Addition of NaOH to First Model Test Solution after BaCl₂ andNa₂CO₃ Addition Simulation ID 8.1. 8.2. 8.3. 8.4. 8.5 Ba : SO₄ (mol:mol)1.00 1.00 1.00 1.00 1.00 CO₃: Ca (mol:mol) 1.10 1.10 1.10 1.10 1.10 OH :Mg (mol:mol) 0.00 1.00 2.00 3.00 4.00 pH 9.18 9.50 9.69 9.87 10.02 MgPrecipitated  0.0% 26.9% 55.9% 74.3% 84.7% Ca Precipitated 99.9% 99.9%99.9% 99.9% 99.9% Ba Precipitated 97.6% 97.6% 97.1% 96.0% 95.1% SO₄Precipitated 97.6% 97.5% 97.1% 96.0% 95.1%

TABLE 15 shows that calcium precipitation is unaffected by the additionof the sodium hydroxide solution but that barium sulfate is somewhatsoluble at an elevated pH, falling from 97.6% precipitation at a pH of9.18 to 95.1% precipitation at a pH of 10.02. Excessive addition ofhydroxide should therefore be avoided to minimize re-dissolution ofbarium sulfates. In practice, the amounts of barium chloride, sodiumcarbonate, and sodium hydroxide will depend on the concentrations ofsulfate, calcium, and magnesium ions present in the MEG-water stream.For example, a lower efficiency for magnesium removal may be acceptableto the operator if the concentration of magnesium ions in the MEG-waterstream is also low.

Second Model Test Solution

The addition of barium chloride to the second model test solution isshown in TABLE 16. TABLE 16 shows that near-quantitative sulfate removalis achieved at Ba²⁺:SO₄ ²⁻ at 1.2:1 mol:mol. Barium precipitation atthis Ba²⁺:SO₄ ²⁻ ratio falls from 100% since the barium is present inexcess.

TABLE 16 Addition of BaCl₂ to Second Model Test Solution Simulation ID9.1. 9.2. 9.3. 9.4. 9.5. 9.6. Ba:SO₄ (mol:mol) 0.00 0.26 0.51 0.85 1.021.20 pH 6.037 6.036 6.034 6.032 6.031 6.031 Ca Precipitated  9.4%  9.4% 9.4%  9.4%  9.4%  9.4% Ba Precipitated %   0%   99%   99%   99%   95%  83% SO₄ Precipitated %   0%   25%   51%   85%   97%   99% CO₃Precipitated % 38.5% 38.6% 38.6% 38.6% 38.7% 38.7%It should be noted that some calcium precipitation is observed under thestarting conditions due to the conversion of calcium bicarbonate tocalcium carbonate as per the reaction below:

Ca²⁺(aq)+2HCO₃ ⁻(aq)→CaCO₃(s)+CO₂(aq)+H₂O

TABLE 17 shows the extent of solids precipitation after the addition ofsodium carbonate to the second model test solution (Ba²⁺:SO₄ ²⁻ at 1.2:1mol:mol).

TABLE 17 Addition of Na₂CO₃ to Second Model Test Solution after Additionof BaCl₂ Sample ID 10.1. 10.2. 10.3. 10.4. 10.5. 10.6. Ba²⁺:SO₄ ²⁻(mol:mol) 1.200 1.200 1.200 1.200 1.200 1.200 Added CO₃ ²⁻:Ca²⁺(mol:mol) 0.000 0.483 0.725 0.967 1.047 1.128 pH 6.031 6.211 6.407 7.1818.044 8.737 Mg Precipitated — — — — — — Ca Precipitated  9.4% 56.8%79.7% 98.3% 99.8% 99.9% Ba Precipitated 82.9% 83.0% 83.1% 83.1% 82.9%98.1% SO₄ Precipitated 99.4% 99.6% 99.7% 99.7% 99.5% 97.3% CO₃Precipitated 38.7% 78.1% 82.3% 81.2% 77.3% 72.9%TABLE 17 shows that near-quantitative calcium precipitation is achievedas the sodium carbonate to calcium molar ratio is elevated to 1.13(mol:mol) while re-dissolution of sulfate occurs only to a relativelysmall extent. Excessive addition of sodium carbonate elevates thesolution pH to a level where barium sulfate begins to re-dissolve.

TABLE 18 shows the effect of sodium hydroxide addition to the solution10.5. shown in TABLE 17.

TABLE 18 Addition of NaOH to Second Model Test Solution after Additionof BaCl₂ and Na₂CO₃ Sample ID 11.1. 11.2. 11.3. 11.4. 11.5. Ba²⁺: SO₄ ²⁻1.200 1.200 1.200 1.200 1.200 (mol:mol) Added CO₃ ²⁻: 1.047 1.047 1.0471.047 1.047 Ca²⁺(mol:mol) OH⁻ : Mg²⁺ 0.000 2.380 3.570 4.760 7.140(mol:mol) pH 8.044 9.533 9.778 9.911 10.154 Mg Precipitated  0.0%  0.0% 31.4%  43.1%  76.1% Ca Precipitated 99.8% 100.0% 100.0% 100.0% 100.0%Ba Precipitated 82.9%  99.2%  99.4%  99.5%  99.6% SO₄ Precipitated 99.5% 92.8%  90.2%  87.8%  84.1% CO₃ Precipitated 77.3%  77.4%  78.5%  78.6% 78.7%TABLE 18 shows that magnesium precipitation requires elevated pH butthat at pH of 9.5 and above re-dissolution of barium sulfate occurs,which leads to increased levels of sulfate in the resulting solution.Simultaneous quantitative precipitation (100%) of calcium, magnesium,and sulfate by the addition of carbonate, hydroxide, and barium ionscannot be achieved so the operator must consider the composition of theMEG-water stream as well as the absolute quantities and percentages ofsulfate, calcium, and magnesium to be removed. Some carry-through ofpotentially scaling calcium, magnesium, and sulfate ions may occur, andcan be managed through a combination of clean-in-place (CIP) systems andaddition of appropriate scale inhibitors.

It should be noted that the barium chloride and sodium carbonatesolutions should not be pre-mixed before addition to MEG-water streamssince precipitation of barium carbonate may occur as shown by thereaction below:

BaCl₂(aq)+Na₂CO₃(aq)→BaCO₃(s)+2NaCl(aq)

However, in the presence of calcium ions and sulfate ions (i.e., in thechemical treatment tank), barium sulfate and calcium carbonate will bethe preferred solid precipitants.

An advantage of the present invention is that it removes divalentcations and sulfate from MEG-water streams in order to improve theefficiency of MEG reclamation or MEG regeneration. The present inventionalso minimizes the formation of scale inside pipelines and processequipment, thereby improving equipment availability. Other advantagesare that the present invention reduces the need for the use ofclean-in-place systems and scale inhibitors and reduces the amount oftime that the process equipment must be taken off-line for cleaning.

While preferred embodiments of a system and process for removingdivalent ions from MEG-water streams have been described in detail, aperson of ordinary skill in the art understands that certain changes canbe made in the arrangement of process steps and type of components usedin the system and process without departing from the scope of thefollowing claims.

1-20. (canceled)
 21. A process comprising: routing a monoethylene glycol(MEG) water stream containing sulfate ions to a vessel; adding bariumchloride to the MEG-water stream; and forming and precipitating bariumsulfate in the MEG-water stream to form a treated MEG-water stream. 22.The process of claim 21, further comprising adding a solution of sodiumcarbonate, a solution of sodium hydroxide, or both to the MEG-waterstream to precipitate calcium, magnesium or both.
 23. The process ofclaim 21, further comprising discharging the treated MEG-water streamfrom the vessel, recycling a first portion of the treated MEG-waterstream to the vessel, and routing a second portion of the treatedMEG-water stream to a solids removal system.
 24. The process of claim21, further comprising routing the treated MEG-water stream to a solidsremoval system, separating barium sulfate solids in the solids removalsystem, and recycling the treated MEG-water stream to the vessel afterit passes through the solids removal system.
 25. The process of claim21, further comprising routing the treated MEG-water stream to a solidsremoval system and then to a filtrate tank.
 26. The process of claim 22,wherein the solution of sodium carbonate, the solution of sodiumhydroxide, or both, and the barium chloride are added simultaneously.27. The process of claim 22, wherein the barium chloride and one or bothof the solution of sodium carbonate and the solution of sodium hydroxideare all added sequentially.
 28. The process of claim 26, furthercomprising recycling the treated MEG-water stream to the vessel untilthe precipitation reactions are complete.
 29. The process of claim 24,further comprising adding a solution of sodium carbonate, a solution ofsodium hydroxide, or both to the recycled MEG-water stream toprecipitate calcium, magnesium or both.
 30. The process of claim 29,wherein a solution of sodium carbonate and a solution of sodiumhydroxide are added sequentially to the recycled MEG-stream andresulting solids are separated by solids removal.
 31. A processcomprising: performing a barium chloride treatment by adding bariumchloride to a MEG-water stream containing sulfate ions; and forming andprecipitating barium sulfate in the MEG-water stream to form a treatedMEG-water stream.
 32. The process of claim 31, further comprising addinga solution of sodium carbonate, a solution of sodium hydroxide, or bothto the MEG-water stream to precipitate calcium, magnesium or both. 33.The process of claim 31, further comprising removing solids from thetreated MEG-water stream to form a solids-depleted stream and repeatingthe barium chloride treatment on the solids-depleted stream until allsulfate ions are removed.
 34. The process of claim 31, furthercomprising removing solids from the treated MEG-water stream to form asolids-depleted stream and adding a solution of sodium carbonate, asolution of sodium hydroxide, or both to the solids-depleted stream toprecipitate calcium, magnesium or both.
 35. The process of claim 32,wherein the solution of sodium carbonate, the solution of sodiumhydroxide, or both, and the barium chloride are added simultaneously.36. The process of claim 32, wherein the barium chloride and one or bothof the solution of sodium carbonate and the solution of sodium hydroxideare all added sequentially.
 37. The process of claim 31, furthercomprising removing solids from the MEG-water stream to form asolids-depleted stream, and adding a solution of sodium carbonate, asolution of sodium hydroxide, or both to the solids-depleted stream toprecipitate calcium, magnesium or both.
 38. The process of claim 37,wherein the solution of sodium carbonate and the solution of sodiumhydroxide are both added, with solids removal between addition of thesolution of sodium carbonate and addition of the solution of sodiumhydroxide.
 39. A process comprising: routing a MEG-water stream to avessel; performing a barium chloride treatment by adding barium chlorideto a MEG-water stream containing sulfate ions; forming and precipitatingbarium sulfate in the MEG-water stream to form a treated MEG-waterstream; removing solids from the MEG-water stream to form asolids-depleted stream; recycling the solids-depleted stream to thevessel; and adding a solution of sodium carbonate, a solution of sodiumhydroxide, or both to the solids-depleted stream to precipitate calcium,magnesium or both.
 40. The process of claim 39, wherein the solution ofsodium carbonate and the solution of sodium hydroxide are both added,with solids removal between addition of the solution of sodium carbonateand addition of the solution of sodium hydroxide.